>Suppose this two queries:
>
>select * from chequescaixa where id='888516'
>Returns one Row
>
>[select * from caixamovimento where docid='888516'] &&[docid] refers to [id] in chequescaixa table
>Returns Nothing
>
>[select * from chequescaixa where id not in(select docid from caixamovimento)]
>Returns Nothing

The query will return no rows in 2 cases:
1. All IDs from 'chequescaixa' are present in 'caixamovimento'
2. There're records with NULLs in the 'docid' column in 'caixamovimento'


>But
>
>[select * from chequescaixa where id not in(select docid from caixamovimento where docID IS NOT NULL)]
>Returns one Row


See 2. above


>I supposed the query
>
>[select * from chequescaixa where id not in(select docid from caixamovimento)]
>
>should return the correct value.Is there anything I'm missing?


See 2. above. What result do you expect?